The present invention relates to a method for coupling optical energy out of an optical fiber, in particular, for fiber optical power monitoring used in mainly in the field of optical communication, optical fiber laser and optical fiber sensors.
Optics fibers have been widely used for increasing telecommunication speed and efficiency, where information is transmitted as light signals through light-transmitting glass or silica fibers. Specially designed optic fibers are also used as fiber optic sensors and fiber optic lasers. They are especially advantageous for long-distance communications, because light propagates through the fiber with little attenuation compared to electrical cables.
Optical performance monitoring is critical in providing and maintaining reliable optical networks. FIG. 1 shows some of the many aspects of monitoring light transmissions in the optical fibers. With respect to a signal dropout, it is necessary to immediately identify the optical signal and the place where the dropout has occurred. It is also necessary to check the signal intensity as well as the network connection status.
The traditional method for monitoring optical signals is through tapping the optical fibers by an optical coupler where some optical signals are taken out to be measured by a photo-diode detector (FIG. 2 for the concept). Fiber optical monitors are thus generally used for tapping partial light from the main transmission fiber in fiber optic circuitries. These monitors check the signal intensity as well as the existence/nonexistence of the connection for the optical signal. Fiber optical monitors are indispensable for the construction of highly reliable optical communication system. Optical tap monitors play key role in auto maintaining the optical signals amplifying ratio for an erbium doped fiber amplifiers.
In a traditional optical coupler, the input signal optical fiber is spliced into two or more connections at the coupler (FIG. 3A). But this requires many numbers of mounting steps, resulting in large size, high optical loss, and high cost. It may also involve breaking the fiber into the air and re-coupling back into the fiber are also high loss and high cost. Several alternative improvements in attempting to make fiber inline tapping are made. For example, U.S. Pat. No. 7,116,870 B2, describes an optical monitor 100 that taps light from the main transmission fiber with two microbends 104 and 122 formed on the fiber using CO2 laser radiation (FIG. 3B), the entirety of which is incorporated by reference for technology background information. With the downstream reflecting surface 106 formed in the cladding, main transmission light signals 114 will partially leak out light 116, among which light 120 is further direct to a photodetector by reflecting surface 106. The reflecting surface 106 is a notch created in the cladding by using laser ablation, at an angle of approximately 44 degrees to the perpendicular of the fiber axis to induce a total internal reflection for light incident on the surface. To reduce wavelength dependence of the tapping the two microbends need to be spaced apart by a distance approximately equal to one-half of the intermodal beat length. Although this design achieves fiber inline tap monitoring, it has major drawbacks of compromising fiber integrity due to micro-bending and notch ablation on the fiber. Optic glass fibers are well known to be prone to break upon micro-defects. This method thus requires high standard in precision and less error tolerance during fabrication.
Another fiber inline tap monitor design is disclosed in U.S. Pat. No. 7,412,137 B2, the entirety of which is incorporated by reference for technology background information. As shown in FIG. 3C, instead of bending the fiber, this design leaks out light from the main transmission fiber by fusion splicing two optical fibers with an offset 5 between the core optical axes 15 and 16. Coupled with a reflecting surface 6, tapped light from the off-set is redirected to a photo-diode 7. However, the reflector surface 6 is fabricated onto the fiber cladding by grading a notch on the fiber. To fabricate a notch on tiny optic fiber of only 125 micro-meter diameter remains a challenge. The notches also comprise the integrity of the fiber that can cause fiber to break when it is under environmentally induced thermal expansion and retraction. The reflecting surface introduces wavelength dependence because the tapped light needs to pass several surfaces of different reflection index and each adds optical interference. For instrument background, FIG. 3D shows an example electronic device system of optical monitoring in case of the microbended fibers.
This application discloses an improved method of optical fiber inline tapping and optical monitoring that alleviates the requirements of notching on the fiber.